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You can find novelty throw toys of all shapes and characters these days. These are the small plush toys that repeat a series of sound bites whenever they are bumped. They are also a cheap and easy solution for adding sound to a project.
Simply by removing the bump trigger and replacing it with a transistor we will be able to trigger this module to sound off at our will by sending it a voltage trigger by way of another sensor circuit or directly with a micro-controller.

Step 1: Remove the Sound Module

This is the point of somewhat no return. We are going to have to open R2 like a Ton-Ton.
Find a seam along the backside of the toy near where you can feel the voice module underneath.
Using scissors or a knife separate the seam, work through the cotton batting and pull out the voice box.
The circuitry is encased in a simple box the houses the batteries , speaker and control circuit. (Sorry no pic for this, but it is straight forward)
Remove the screw in the top of the case and the unit will separate. The circuit simply pulls out.

Step 2: Find the "Knock" Detector and Remove

On the circuit board there should be a cylinder that looks like a capacitor but does not have the correct markings. (Mine had no marking that I could make out) This is the component that triggers the sound when the toy is bumped, I will be referring to this as the knock sensor. In the pic above it is the green component in the upper right.
To verify, after you think you have found the knock sensor use a piece of wire and jump across the leads the unit should activate.
Now just desolder this component.

Step 3: Replace Trigger With NPN Transistor

Your knock sensor should be removed.
Place the collector an emitter of the NPN transistor you are using into the board where the knock sensor mounted.
Bend the base pin outward to be used for a later signal.

Step 4: Remove Battery Leads and Connect Pigtail

Snip the battery leads and solder wires to the newly installed transistor base pin, and the battery + and - terminals.
Hot glue everything down to make it robust and you are done. Your voice module is ready to install on your robot, Halloween thingy or your staples easy button.
I'll have to keep an eye out for one. I've been looking for old simple MP3 players too but they are hard to find in our area I think most just get thrown away.
There's a small R2 toy out there with trading cards and some other rubbish. I found it in a home bargains near where I live. It was tricky getting into it but the chip inside was much smaller and easier to fit into the Lego R2.
What kind of expanded sound chip did you end up with? (I think that is the direction I'd like to go with another project i have going but there are so many options.)
This is exactly what I did about a month ago. I sacrificed my plush R2 and installed the sound chip into my lego R2. I then upgraded to a sound chip with more sounds and added LEDs that flash when he talks. Just wish I'd thought on and took pictures for an instructable :-(

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Bio: I work in industrial automation and spend any free time making.
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